The long, hard-breaking eagle putt that defined his round of 7-under 63 Friday had just finished its uphill journey to the back of the cup on the 14th hole.

"The miracle shot," he called it.

Allen said he wanted to lag the ball to within 10 feet. That it went in was a bonus.

"It wasn't about me," he said. "Obviously, something else happened. What do you say? The Lord did it or whatever. It was nice it went in, and it was kind of out of my control."

Just about everything else went right for the 54-year-old Allen, too. His best round of the year put him at 10 under after the second round of the U.S. Senior Open and allowed him to break away from the pack for a five-stroke lead.

The cushion was the largest after 36 holes in the tournament's 34-year history.

"Man, oh, man. The biggest lead?" Allen said with a smile. "So I can blow the biggest lead. Is that what you're telling me?"

Rocco Mediate was Allen's closest pursuer after he shot a bogey-free, 3-under 67. Jeff Sluman also had a 67 and was another shot back.

"We've got to go out and get him," Mediate said. "He ain't going to come back, more than likely."

Allen started the day sharing the lead with six other players. He birdied five of the first eight holes and went out in 30 in hot, breezy conditions at the Omaha Country Club. He hit a rough patch after he made the turn, chunking a ball in the long grass on No. 12, but everything was right after he went birdie-eagle on Nos. 13 and 14.

Allen was short with his birdie putt on the last hole, ending his bid to match the U.S. Senior Open record of 62 by Loren Roberts at Prairie Dunes in 2006. His two-round total of 130 was lowest in tournament history.

"You kind of look at the way things have gone so far," Allen said. "I've chipped in a couple times. I've played some bad shots and kind of gotten away without killing myself. There's too far to go, though. Hopefully, in the end, it's predestined, but I've got a long way to go. Only halfway there."

Allen never won in 394 starts on the regular tour, but he has four victories since joining the Champions Tour four years ago, including the 2009 Senior PGA Championship.

He had top-10 finishes in the U.S. Senior Open in 2010-11 and tied for 33rd last year. His game has been on the upswing lately. He won the Mississippi Gulf Resort Classic in March and came to Omaha with three top-10 finishes in his last four events.

It looked as if Allen and Mediate would match birdies early. Mediate had three in four holes to start the afternoon and took a brief lead.

But Allen, playing in the group behind him, went on a tear. He hit every green in regulation and made the turn with a 3-shot lead.

Allen's strength is his irons, and he was hitting them pure on the front nine, seemingly oblivious to the southerly winds gusting to 25 mph and the course's elevation changes.

The most impressive of his six birdies came on the 477-yard eighth hole, which has been playing as the toughest on the course. He hit a 210-yard 4-iron to 15 feet, then raised his right hand and gave a quick thumbs-up after his straight putt dropped into the cup.

His eagle came after he hit driver-hybrid to the bottom shelf of the two-tiered 14th green. With his long putter anchored to his chest, there was no threat of Allen coming up short on the putt that measured 50-60 feet.

The ball went up the ridge and broke about 20 feet to the left before it hit the back of the cup, popped up and dropped in.

"I could hit that putt 100 times and not get it within 5 feet, I guarantee you," Allen said. "So I hit a good putt. I did what I was trying to do, but then it went in. It's one of those days, things were going right, and that was one of the things that really went right."

Defending champion Roger Chapman was among the players who missed the cut. He followed his first-round 74 with a 76.